1. Field of the Invention
The present invention relates to an improvement of a data transmission system suitable for CATV (cable television) systems.
2. Related Art
In a CATV system, video signals such as signals from VTRs or ITVs and signals of commercial broadcasting programs are transmitted from a sender through a transmission line (for example, a coaxial cable) to terminals such as tuners and video monitors.
It has become the practice in recent CATV systems that in order to cope with the diversification of information, character information blocks produced on a computer are sent to terminals in the form of character codes, along with video signals such as animation signals of the NTSC system, such character information blocks being displayed in color on monitors at the terminals. Such character information blocks are usually transmitted to terminals by assigning appropriate frequency bands in an operating frequency range to the character information blocks and by effecting a frequency-shift-keying or phase-shift-keying modulation of the character information blocks. In order to inhibit a wide frequency band from being occupied, it is common that a plurality of character information blocks are sent serially through one frequency band.
In a CATV data transmission system of the prior art, a plurality of video sources such as VTR, ITV and received signals of commercial broadcasting programs are respectively connected to video modulators to which different carrier frequencies are assigned. The modulated video signals are sent through a transmission line to tuners in receiving terminals. Character information blocks produced on a computer are fed through an adapter to an FSK modulator and sent to the tuners along with the modulated video signals. These video signals and character information blocks are demodulated at the tuners and displayed on monitors at selected channels.
FIG. 1 shows a manner by which character information blocks are serially sent in a CATV system of the prior art. In this figure, each information block D1, D2 . . . or Di includes character information to be displayed on one picture and comprises identification data, code data and property data. If one character picture is of the size of (80 columns).times.(20 rows), the amount of the code data is 1600 bytes and if each property is defined by one byte, the property data is also 1600 bytes. FIG. 1 indicates that a plurality number i of information blocks D1, D2, . . . , Di are sent in one cycle.
At each receiving side, an operator designates a picture number to display, on a monitor, a character information block having the identification data corresponding to the designated picture number. Since the information transmission is effected uni-directionally just as in broadcasting and the character information blocks are received at terminals regardless of the order in which the character informnation blocks are sent, the character information blocks are at all times sent cyclically.
FIG. 2 is a block diagram showing the construction of an adapter 100 used in a CATV system of the prior art. Character information blocks D1, D2, . . . , Di produced serially on a computer are fed through an input interface 102 to a buffer memory 104 in an adapter 100. The stored blocks are sequentially taken out of the buffer memory 104 under the control of a data transmission control unit 106 and sent through an output interface 108 to an FSK modulator.
As could be understood from the above description, the adapter serves to store character information blocks in the buffer memory 104 and cyclically sends the stored character information blocks at a constant period. The computer provides new character information blocks to the adapter only when the updating of character information blocks takes place. It is thus unnecessary for the computer to deal with the control of information transmission, which reduces a load on the computer. On the other hand, it should be noted that, even when the updating of information blocks takes place, the computer is requested to send, as character information blocks for the initial picture, the whole character information blocks for a complete picture including new character information blocks produced when the updating of information blocks takes place.
In a data transmission system of a prior art CATV system, since the order of character information blocks produced on a computer and sent toterminals is fixed even when some character information blocks are requested to be updated, it usually takes some time for new character information blocks to reach a terminal from a sender, which results in a significant delay in the time taken to update information blocks to be displayed at monitors. Such a problem becomes more serious as the number of character pictures increases. For example, it takes 100 milliseconds to transmit a character information block for one picture, and therefore it takes 100.times.100 milliseconds to transmit character information blocks for 100 pictures.
In order to solve this problem, Japanese Patent Public Disclosure No. 157488/87 proposes that each terminal has a means by which character information blocks received at the terminal are stored in a data memory while some information blocks are renewed with the latest ones and the character information blocks stored in the data memory and included in a designated picture are read out from the data memory to be displayed. This can shorten the time taken to display updated information blocks.
Since such a means as described above is provided at a terminal, however, each terminal must have a data extracting circuit, line buffers, data storing RAMs and the like, which makes the circuit construction complicated.